Pooling on flat roof

I'm trying to help a friend that has a problem with pooling water on the flat roof of a garage.

Although the garage is a single of approx 3.2m x 5.85m it is one of a pair (her neighbour owns the other one; total building size approx 6.4m x 5.85m with a 4inch party wall.

The roof construction is 100mm x 50mm regularized timber joists, OSB decking and roll asphalt with granite chipping covering, all of which was replaced 5 to 10 years ago. Can't be more specific on timing due to previous owner and neighbours being elderly.

Each garage roof has a large area of pooling on either side of the party wall. The pooling extends from about 1000mm from the front of the roofs to where it runs off at the rear (in one corner of each) and extends to about 300mm from the side to slightly less over the party wall; it/they are big puddles of water.

The pooling appears to be due to the joists below sagging (a lot). The joists timbers are installed from side wall to party wall (3050mm span), with no additional supports (Not sure of joist spacing or timber class). The joists are either butted together with the neighbours joists over the 4 inch party wall (i.e. 2 inch bearing) or span across the building width of 6.4m (no sign of staggered joists over party wall). The roof is sloped with reducing height firrings between the decking and joists ranging from 50mm at the front to zero at the back.

This situation is obviously not good and will probably lead to premature failure of the roof covering and possible roof structure especially if there is a lot of rain that freezes in the winter and is then topped with significant snow. The pooling water will be in direct sunlight for most of the day

Has anyone any suggestions what can be done?
How long could the roof remain sound given the situation?
What would the likely cost of resolving the issue be, location is north Hampshire.
What would the likely cost be to replace flat roof with pitched roof?
Would a pitched roof replacement require planning permission?

Any help would be much appreciated, many thanks.

Unfortunately the only long term solution is to replace the whole roof inlcuding the undersized joists

Joists should be either 6 x 2 or 7 x 2

if the 4 x 2 joisted roof had been built with a much greater fall it wouldve got away with the undersizing.

I dont know if would work but I suppose in theory if each joist was jacked up with acrows, then sistered with an 8 x 2 joist, then let go, it might eliminate the sagging -it might be worth a try

my guess is that every joist would need jacking up and the joists sistered then all releasing -if you do one at a time the others ones sagging will force the new strong joist down

The sistering timbers dont have to be supported at the end -just left a bit short is fine

Personally I think its worth a go, if the roof asphalt is in good condition.

Notch7

Thanks for the reply. I had considered putting new 6" plus joists next to the existing ones but hadn't thought to cut them short and rely on the existing joist bearing to support the new timber - great idea.

Presumably the sistered joists would need bolting together with star washers in between them, if so, what spacing for the bolts and what size - M10 M12?

I am a little concerned that jacking the old joists up to get a flat deck will put a lot of upwards tension/thrust on the roof causing it to lift at the edges. If this is likely to happen would tieing the old joists to the walls with galvanized restraint straps prevent it and if so what size? See https://www.screwfix.com/p/sabrefix-roll-edge-restraint-strap-bend-500-x-100mm-5-pack/85398.

Presumably I won't need to install firrings on top of the new joist just jack the joists up tight underneath the decking? Obviously this will result in the bottom of the new joists getting lower the nearer they are to the run off end?

Doing a bit of research it would appear that the slope on the roof should have been between 1:40 to 1:80 giving a rise of between 146mm to 73mm at the high end down to nothing at the run off end, is this correct?


On a slightly different matter in a different location, is it normal to have the side facia of a flat roof house extension installed at an angle following the slope of the roof? I thought most if all side facias were horizontal with the roof slope hidden behind it.

wobbly2 said:

old joists up to get a flat deck will put a lot of upwards tension/thrust on the roof causing it to lift at the edges.

Click to expand...

yes good point -Its obvious that would happen.

restraint straps would be ideal for this -you need to get plenty of fixings into masonry quite a few courses down and those sort of straps are perfect.

flat roofs need an effective 1 in 80 fall so water will run off. In practice allowing for a bit of timber sag plus the overlap joints on felt, 1 in 40 is the recommended fall.

So yes on a 5850mm span, a fall of 146mm is needed for a 1 in 40 fall


on your other question: no side fascia are not necessarily horizontal, although its best visually if they are.

if a fascia follows the line of the roof fall then it probably has an upstand to stop water running over the edge -if the roof is felted with a felt upstand then the fascia may just butt to the underside of that and it would be sloping.

A fascia is only a decorative detail hiding the roof joist end -so there is no right or wrong.

Thanks for the info re the flat roof.

Your right about the extension side facia not looking so good if angled. In the case I asked about the extension extends about 4 metre from the back of a corner plot house with a 400mm ish deep facia that's facing my lounge window and it looks bl***y awful, especially as it now having and 1.5m x 4m fully glazed pyramid roof lantern sat on a 300mm high plinth in the middle of it, which will stick up like a lighthouse dominating the street scene.